This invention relates to a tap for monitoring an optical signal in a fiber. The tap finds particular application for monitoring the output power of optical transmitters.
Optical fiber waveguide transmitters are often required to be equipped with a transmitted power monitor. The most common application of such a monitor is that of alarm diagnosis in the event of optical source failure. Other applications include the stabilization of output power and feedback linearization.
Basically, in a fiberoptic monitoring tap, a predetermined fraction of light from an optical fiber is diverted from the fiber and directed to a photodetector where its power level is measured.
A known fiberoptic tap is made by twisting together two lengths of optical fiber, heating the twisted pair in a twist region, and pulling the twisted pair from either end to encourage fusion at the heated region. The resulting component has four ports. Typically, when monitoring transmitter power, one port is coupled to the optical source, a second port is spliced to the output fiber, a third port is redundant and so is placed in a reservoir of index-matched oil, and from the last port is taken the monitored light signal from which the transmitted output power can be calculated.
Like other known taps, this power tap incurs a penalty in the form of reduction of transmitted power. Some power is diverted to the monitor photodetector and some is lost by device coupling imperfections.